Numerous past studies with rat pairs in parabiotic circulation, transplanted hepatic tissue and transplanted hepatocytes have shown that liver regeneration after partial hepatectomy is associated with the appearance in the rat serum of factors that stimulate growth of hepatocytes. The nature of these growth factors and their importance to liver regeneration and chemical carcinogenesis has not been elucidated due to lack of a suitable in vitro bioassay. Other recent studies have shown that the neoplastic transformation induced by chemical carcinogens is associated with the production of transforming growth factors. We have defined the conditions that are required to allow rat parenchymal hepatocytes in primary culture to respond to growth factors that are present in the rat serum. The effect of partially hepatectomized rat serum is consistently higher than that of control rat serum. We have recently shown that the activity responsible for the increased growth stimulation by the hepatectomized rat serum is not due to differences in the levels of insulin, EGF, PDGF, hydrocortisone, triiodothyronine or glucagon. A possible involvement of catecholamines is suggested but the exact nature of involvement is not clear. Using serum fractionation techniques we found that the hepatopoietic activity of the rat serum is due to the synergistic effects of two factors, (empirically defined as Hepatopoietins A and B) of large and small molecular weight respectively. The purpose of this project is to: (1)\purify, isolate and characterize the Hepatopoietins; (2)\measure these substances to determine their levels during hepatic regeneration and their sources of origin; (3)\determine the growth stimulatory effects of these factors and compare their effects on normal hepatocytes and cells from hepatocellular carcinomas initiated by chemical carcinogens; (4)\examine the possibility that the neoplastic nature of the hepatocellular carcinomas may be dependent on either abnormal response to Hepatopoietins or abnormal production by hepatocarcinomas of these factors or factors which mimic the Hepatopoietin effect.